The present invention relates to a cooling device for preventing high temperature deterioration of a liquid crystal and a polarizing plate contained in a liquid crystal panel which is used in an optical device such as a liquid crystal projector.
FIG. 23 shows schematically a liquid crystal projector. Light Ls emitted from a light source L has components of blue, red and green. The light Ls reflected by a concave mirror CM is changed to parallel beams by a condenser lens CL. Blue light Lb is separated from the light Ls by a dichroic mirror DMb1, and red light Lr is separated by a dichroic mirror DMr1, and remaining green light Lg goes straight.
The blue light Lb is bent at a right angle by the dichroic mirror DMb1 and further bent at a right angle by a mirror Mb1. The red light Lr is bent by mirrors DMr1 and Mr1. Thus, the blue light Lb, red light Lr and green light Lg go straight in parallel.
In the respective light paths, transparent liquid crystal panels LCb, LCr, and LCg are provided for corresponding colors, so that light beams Lb, Lr and Lg are changed to transmitted light beams lb, lr and lg each having image information of the respective color. The blue light lb is reflected by a mirror Mb2 and a dichroic mirror DMb2 and enters in the path of the green light lg. The red light lr is reflected by a mirror Mr2 and a dichroic mirror DMr2.
Thus, all transmitted light beams lb, lr and lg become a light ls. The light ls is projected on a screen S through a lens PL.
In such a projection system, the liquid crystal panels LCb, LCr and LCg are heated by the projected light at a high temperature. If liquid crystal and polarizing plates in the liquid crystal panel become high temperature, their characteristics are deteriorated. Therefore, it is necessary to provide a cooling device for cooling the liquid crystal panels.
FIG. 24 shows an air cooling system for liquid crystal panels. In the system, liquid crystal panels LCb, LCr and LCg are mounted on a base plate SP at respective openings Wb, Wr and Wg formed in the base plate. The liquid crystal panels are cooled by cooling air CW from a blade FB of a cooling fan CF.
Temperature decrement .DELTA.T with respect to ambient temperature is expressed as follows. EQU .DELTA.T=Q/.alpha.A
where Q is the heat quantity of the cooled body, A is the surface area of the cooled body, .alpha. is the heat transfer rate between air and the cooled body.
The heat transfer rate .alpha. is expressed as follows. EQU .alpha.=.lambda./L.times.0.66Pr.sup.1/2 .times.(UL/v).sup.1/2
where .lambda. is the heat conductivity, L is the distance, Pr is the Prandtle number, U is the flow rate of air, v is a coefficient of dynamic viscosity.
Therefore, the cooling efficiency for the liquid crystal panels LCb, LCr, and LCg increases with the flow rate of the cooling air CW theoretically. However, when the flow rate of the cooling air exceeds a certain value, the cooling efficiency does not increase in proportion to the flow rate.
FIG. 25 shows the decrease of surface temperature of a polarizing plate with respect to the increase of flow rate of cooling air. From the graph, it will be seen that the temperature of the polarizing plate hardly decreases when the flow rate exceed 1.0 m/sec..
The inventors of the present invention have proposed to cool the liquid crystal and the polarizing plate with cooling liquid having a large heat capacity, for example in Japanese Utility Model Laid Open 60-136045.